Majormd pretty much covered this, but to emphasize:
Tryptophan and tyrosine and any number of other amino acids are used by your body to make more neurotransmitters. They are simply converted into the required neurotransmitter whenever your body needs it.
Nicotine and other drugs operate on two levels: they elevate the demand for neurotransmitters and they act as neurotransmitters themselves. The result of both is that your brain becomes accustomed to the altered levels of neurotransmitters floating around; and if you quit cold turkey, your body wants those neurotransmitters back.
Its all bodily economics: Excess supply has little effect unless there is a demand for it.
As for turkey and all that jazz, I’m inclined to believe (though this is a bit of a WAG) that the tryptophan, which is a melatonin (and melanin) precursor, is not necessarily the cause of the thanksgiving nappies. Lots of other foods, such as bananas, have as much, if not more, tryptophan in them. No one wants to take a nap after eating a bunch of bananas.
Jason R Remy
“One pill makes you taller, and one pill makes you small, but the ones that mother gives you don’t do anything at all”
– Jefferson Airplane * White Rabbit * (Slick, G. 1966)